It is known practice to associate with digital data representing a content, in particular a video or audio sequence, copy control information items commonly denoted CCI (standing for “Copy Control Information”) or else CGMS (standing for “Copy Generation Management System”).
These information items, which are inserted into the data by the content provider, generally define four possible states for the data:                copying authorized (“copy free”);        a single generation (or a given number of generations) of copying authorized (“copy once” or “copy N times”);        no more copying authorized (“copy no more”);        copying never authorized (“copy never”).        
When the data belong to the last two categories, they may not be copied. That is to say they may only be viewed and/or listened to, when dealing with video and/or audio data, for example but they may not be recorded, or if a recording is made illicitly, it must not be possible to reuse it later.
A first approach for guaranteeing this result consists in having any recording apparatus verify the aforesaid control data and, should data whose copying is unauthorized be detected, in disabling the recording.
However, this type of approach has restrictions since it can only operate with compliant (non “pirated”) recording apparatus.
Moreover, another method has been proposed in order that, when data are broadcast in a digital network such as a domestic digital network, they can only be copied within the network. To do this, the data which are broadcast in the digital network are scrambled with control words and these control words are encrypted with the aid of keys specific to the digital network around which the data flow. Thus, if copies of these data are made, they can only be played back within the digital network in which they have been copied. Refer to the French Patent Application of THOMSON multimedia, published as No. FR-A-2 792 482, for further details with regard to this matter.
However, this method does not allow the complete prevention of copying. There are in fact cases where content providers desire data to be broadcast in “live” in a digital network but do not want it to be possible to make copies in order to replay this content later in the network. A typical example relates to the broadcasting of films by digital television operators.